A crystal of calcium phosphate (hydroxyapatite) formed when a calcium aqueous solution is mixed with a phosphoric acid aqueous solution so that a supersaturation state is obtained has a property to bond with DNA. A method in which calcium phosphate and DNA are coprecipitated making use of this property has been widely utilized as a method for introducing DNA into a cell. The problems of this method include a very narrow range of an optimum condition, which makes handling thereof difficult, and less liability to obtain reproducibility. In particular, growth in the crystal of calcium phosphate is very fast to form the giant crystal, and therefore it is pointed out that an efficiency of introducing DNA is reduced if a calcium aqueous solution and a phosphoric acid aqueous solution are not quickly worked on a cell after mixing. Also, hydroxyapatite is used as a base substance (for example, a carrier for chromatography) for adsorbing a biologically active substance including polynucleotide such as DNA, other peptides or polypeptide.
In the foregoing method for introducing DNA (or gene) into a cell, development of a method for controlling a growth in a crystal of calcium phosphate and a particle diameter thereof is considered to be important to a rise in an introducing efficiency of DNA, a reproducibility and a storage stability. Further, if substances carried or adsorbed on such crystal can widely be used as various biologically active substances without being restricted to DNA described above and a particle diameter of a particle formed can be controlled, it is considered to be important in providing a carrier system which can widely be used for carrying medicines or a delivery system for medicines. Accordingly, an object of the present invention is to provide a composition useful for forming particles which not only enhance an efficiency of introducing DNA into a cell but also have a wide and controlled particle diameter and which can stably carry a biologically active substance and conveniently deliver the biologically active substance to a target cell, a desired tissue or a local site.